In selenium-containing wastewater mentioned above, selenium is normally dissolved in wastewater in the form of selenate ions (SeO.sub.4.sup.2- : hexavalent selenium) and/or selenite ions (SeO.sub.3.sup.2- : tetravalent selenium). In some cases, only one of these kinds of ions may be present in wastewater, but in many cases the two types coexist. Technology for removing selenium from selenium-containing wastewater has not been sufficiently studied, since removal of selenium in particular had not been required before the Water Pollution Control Law was revised in 1993 to add selenium to regulated items. According to the revision, the selenium concentration of wastewater must be lower than 0.1 mg Se/L.
Under the circumstances, there have been studied the following methods (1)-(3) for removing selenate ions and selenite ions (hereinafter, may be collectively referred to as soluble selenium) from selenium-containing wastewater.
(1) Metal salts such as a magnesium salt, a zinc salt, a ferric salt, etc. are added to selenium-containing wastewater to remove selenium in the form of insoluble compounds of selenium with these metal salts.
(2) A ferrous salt, which can reduce soluble selenium to simple selenium (Se), is added to selenium-containing wastewater so as to make soluble selenium insoluble. Subsequently, the thus-generated insoluble substances are removed through sedimentation or the like. In this method, soluble selenium is reduced to simple selenium, which is insoluble, through reducing action of Fe.sup.2+ ; Fe.sup.2+ reacts with soluble selenium to form insoluble selenium compounds; and generated hydroxide flocs cause simple selenium or soluble selenium to undergo various inter reactions such as coprecipitation, separation, and the like. Thus, soluble selenium is made into insoluble substances, which are then removed.
(3) Soluble selenium contained in selenium-containing wastewater is reduced to simple selenium through anaerobic biological treatment. Thus-formed simple selenium is separated and removed.
However, the above methods (1)-(3) had the following shortcomings. In the chemical treatment method (1), in which metal salts such as a magnesium salt, a zinc salt, a ferric salt, etc. are added to form insoluble selenium compounds, insoluble selenium compounds are formed relatively easily from tetravalent selenite ions, whereas insoluble selenium compounds are less likely to be formed from hexavalent selenate ions. Accordingly, hexavalent selenate ions remain within treated water. Thus, removing selenate ions was difficult.
In the chemical treatment method (2), in which a ferrous salt is added to make soluble selenium into insoluble substances, both selenate ions and selenite ions can be removed. However, this method requires a considerably large amount of a ferrous salt, and a large amount of an insoluble substance (sludge) is generated, thus involving a disadvantage in terms of running cost and waste (sludge) treatment cost.
In the biological treatment method (3), in which soluble selenium is reduced to simple selenium through anaerobic biological treatment, both selenate ions and selenite ions can be removed through reduction to simple selenium. However, it is difficult to remove selenium to a trace level. Thus, it was often difficult to clear the new wastewater criterion that limits selenium concentration to lower than 0.1 mg Se/L. For wastewater having a low selenium concentration, the selenium concentration may be reduced to below the allowable limit through anaerobic biological treatment alone. However, according to a study conducted by the inventors of the present invention, in the case of wastewater which contains a relatively high concentration of selenium, or wastewater whose selenium concentration varies, biological treatment became unstable due to load variations and like causes, and thus it was difficult to perform stable treatment by which the selenium concentration was always being held below the allowable limit.
The present invention has been achieved in view of the foregoing circumstances, and an object of the present invention is to provide a cost-effective method for treating selenium-containing wastewater capable of removing hexavalent selenate ions and/or tetravalent selenite ions from selenium-containing wastewater so as to satisfy a wastewater regulation limit of lower than 0.1 mg Se/L, without using a large amount of chemicals such as metal salts and the like and without generating a large amount of sludge, and to provide an apparatus for carrying out the method.